The present invention relates generally to the automatic control of electric current flow through the windings of an electromagnet, and, in particular, to electronic demagnetization of magnetic chucks and the like.
Ferromagnetic material is characterized by a plurality of individually polarized magnetic domains. When the domains are all randomly oriented, the material will exhibit no net magnetism since the individual contributions of the domains cancel. On the other hand, when many domains are oriented in the same direction, the material acts as a magnet and is said to be magnetized. A ferromagnetic body may be magnetized by passing a direct electric current through coils wound around the body. The magnitude of the current determines the strength of the induced magnetization up to the point of saturation, at which all of the domains are similarly oriented.
Ferromagnetic materials exhibit a residual or permanent magnetism called hysteresis after the current inducing the magnetization is turned off. That is, while some of the domains lose their common orientation, others retain the induced polarity in the absence of the electrically generated magnetic field. Certain ferromagnetic materials exhibit this phenomenon to a far greater extent than others. Soft iron with high magnetic permeability, for example, retains far less residual magnetism than steel, for example.
The permanently magnetized material is difficult to demagnetize. Demagnetization requires a return to the randomly oriented domain condition. An electrically generated magnetic field has the opposite tendency: it tends to cause alignment of the domains. Thus, the material cannot be demagnetized by simply removing the current. Nor can it be demagnetized by applying a current in the opposite direction, as this would leave the material with residual magnetism in the opposite direction.
It has been found that for materials which exhibit substantial residual magnetism, demagnetization can be performed by using a sequence of successive reversals of current in the electrical windings while decreasing the current at each reversal. In this manner, a progressively smaller percentage of domains have their orientations switched back and forth. The result approximates a re-randomization of the orientation of the domains.
The many applications where electromagnets are used to attract or hold another ferromagnetic element or workpiece include magnetic cranes, chucks, clutches, molding apparatus, etc. Magnetic chucks for industrial abrasive grinding apparatus present an application of particular significance. In vertical rotary surface grinders of the type manufactured by the Cone-Blanchard Machine Company of Windsor, Vt., the assignee of the present application, the workpiece to be flat-ground is affixed to a rotating horizontal table. The workpiece is abraded by an overhead counter rotating grinding wheel exactly parallel to the surface of the rotating table. The table comprises a solid disc of steel with a perfectly flat polished horizontal surface. Coaxial electrical windings beneath the disc cause the entire rotating table to act as an electromagnet so as to attract and securely hold the ferromagnetic workpiece or workpieces to the top of the table while being ground. The table is thus referred to as a rotary magnetic chuck and is capable of holding large and small workpieces of any configuration. Typical magnetic chucks in use today vary in diameter from sixteen inches to one hundred twenty inches.
The strength of the magnetic field should be adjustable to accommodate thick or thin workpieces and this can be relatively easily accomplished by providing adjustable direct current levels for the electromagnet windings. While requiring high magnetization on the one hand to hold rotating workpieces against the frictional and centrifugal forces experienced during the grinding process, the chuck must also be capable of substantially complete demagnetization in order to hold, reposition or remove the workpieces. Since the hardened steel used for the best magnetic chucks exhibits a significant degree of residual magnetism, thorough demagnetization becomes a difficult problem, particularly on the ten-foot diameter chucks.
In the past the successive reversal, decreasing current step technique has been used for magnetic chucks. In this type of control apparatus, the electric current is decreased in a sequence of steps by motor-driven switches which select taps on the transformer that powers the rectifier circuitry. The motor-driven switches include contacts that reverse the polarity in alternate steps. Since the load, i.e. the electromagnet windings, is inductive, each step involves a long exponential decay followed by a long exponential rise in current. The inductance associated with the windings for the larger chucks is high. And, since the voltage is reduced at each successive reversal, the time required to drive the current to the desired value at each step does not decrease substantially. Thus in such devices the timing of each step is usually fixed, and as a result each step must be excessively long to accommodate the effects of inductance. The prior device provides a demagnetization cycle which lasts on the order of a half a minute with the larger magnetic chucks. During this interval, the operator must wait before he can reposition the workpiece. On jobs calling for numerous repositionings of the workpiece, this delay can accumulate to the point where it substantially affects the production rate that this machinery can achieve. In summary, with known devices, it appears that the speed of the demagnetization cycle is constrained by (1) mechanical switching, (2) a fixed cycle and (3) inability to use as large a drive voltage as possible.
Furthermore, changing the number of reversals and/or the size of the current decrements at each step involves lengthy mechanical resettings or the availability of several devices set up for differing cycles or sequences. The cost of the relatively expensive transformer components and number of mechanical switches militates against these expedients.